Forget the white/blue dress, is this seaweed blue or red?

(Image&colon; Juliet Brodie/The Trustees of the Natural History Museum)

Just like soap bubbles, some seaweed can produce extraordinary colours. On a bright, sunny day, the tips of this edible red alga can become an other-worldly blue.

Now Juliet Brodie from the Natural History Museum in London and her team have figured out how this Irish moss, Chondrus crispus, does it.

By taking detailed images of the plant under a microscope, they found that the outer coating of the tips of the fronds consists of several transparent layers. When submerged in water, these reflect blue light, giving the moss its shine (see photo below, left). Nearer the base of the frond, the protective “skin” is thicker, so it loses its iridescence, as shown on the right.

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(Image&colon; Chandler et al. (2015))

The role, if it even has one, of the seaweed’s lustre is still a mystery. It may be that it deters would-be seaweed munchers, when fresh and hydrated, by displaying an uncharacteristic blue colour. Or it could act as a sunscreen by reducing the absorption of UV radiation from the sun. When growing underwater, the extremities of the seaweed are most vulnerable. “The tips of fronds grow towards the surface so they are more vulnerable to UV light and grazing,” says Brodie.

The world’s only iridescent mammal, a blind mole that lives underground, is thought to have accidentally evolved its shine, although it helps repel water from its hairs.

A rare iridescent fossil beetle uncovered a few years ago had retained its shimmer for 49 million years.